Such a control arrangement can in particular be used advantageously to regulate the toner concentration and the fill level in a mixing unit (having at least toner and carrier fluid) in the developer station of an electrographic printing apparatus. Assuming this use case the control arrangement of this application is therefore described without the control arrangement being limited to this use case.
For single color or multicolor printing of a printing substrate (for example of a single page or of a belt-shaped recording material made of the most varied materials, for example paper or thin plastic or metal films), it is known to generate image-dependent charge images on a charge image carrier (for example a photoconductor), the image-dependent charge images corresponding to the images to be printed, and comprised of regions to be inked and regions that are not to be inked. With a developer station the regions of the charge images that are to be inked are revealed on the charge image carrier via toner as toner images. The toner image that is thereby generated is subsequently transfer-printed onto a printing substrate and fixed there in a transfer printing zone.
A developer fluid having at least charged toner and carrier fluid can thereby be used to ink the charge images. Possible carrier fluids are hydrocarbons, silicone oils and others.
A method for such an electrophoretic printing in digital printing systems is known from WO 2005/013013 A2 (US 2006/0150836 A1, DE 10 2005 055 156 B3), for example. After the charge images of the images to be printed have been generated on the charge image carrier, these are inked with toner into toner images by a developer station. Here carrier fluid containing silicone oil, with dye particles (toner) dispersed in it, is thereby used as a developer fluid. The feed of the developer fluid to the charge image carrier can take place via a developer roller to which the developer fluid is supplied by a raster roller on which a chamber blade is arranged. The toner images are subsequently accepted from the charge image carrier by a transfer unit and transferred onto the printing substrate in a transfer printing zone.
In the developer station (for example in a mixing unit) the developer fluid used in the printing apparatus can be mixed together from a toner concentrate having toner and carrier fluid and from carrier fluid. The toner concentrate and the carrier fluid can respectively be contained in reservoirs and can be regulated in the printing operation, for example they can be transported into the mixing unit by means of pumping. For a proper print image it is necessary that enough toner is contained in the carrier fluid so that the toner concentration in the developer fluid has the provided value. It must thereby be taken into account that in the printing operation carrier fluid is continually taken from the mixing unit and partially applied to the printing substrate. The remaining developer fluid with lower toner concentration that was not used for the printing can be supplied to the mixing unit again or can be discarded in a waste container.
The toner concentration and the fill level in the mixing unit change due to the continuous removal and resupply of developer fluid or in particular of toner. However, both variables (toner concentration and fill level) should be kept constant at predetermined desired values via regulation. This can take place solely via the feed of the aforementioned components, namely toner concentrate (toner and carrier fluid) and carrier fluid. The problem thereby exists that the goals to be achieved—namely adjustment of the desired values for the toner concentration and for the fill level in the mixing unit—are coupled with one another and mutually influence one another. For example, the increase of the toner concentration by supplying toner concentrate also leads to an increase of the fill level. Or the decrease of the toner concentration by supplying carrier fluid leads to an increase of the fill level.
An additional problem is to be considered: for the print quality the toner concentration in the mixing unit is decisive (the fill level is of subordinate importance for this); accordingly it should be possible to preferentially adjust the toner concentration in the mixing unit.
An electrographic printing apparatus that has a mixing unit and reservoir for toner concentrate, carrier fluid and charge control substances in the developer station is known from U.S. Pat. No. 5,003,352. The fluids are transported into the mixing unit with the aid of pumps. The fill level is adjusted with a 2-point regulation. Limit value switches for different fill levels serve as sensors. The toner concentration is determined via optical transparency measurement and is readjusted by a separate 2-point regulator. The conductivity in the mixing unit is measured with the aid of electrodes and is regulated with a separate 2-point regulator. All three regulations operate independent of one another. There is no preference given to any control goal.
One method to mix multiple fluids is known from U.S. Pat. No. 3,608,869. The fluid level is regulated via 2-point regulators. The proportions of the individual fluid components to be mixed result from the activation durations of the respective feed pumps. There is no sensor that measures the mixture ratio; and a regulation of the mixture ratio is not provided.